Nowadays, repeater systems are constituted to serve a wide frequency band, for example a complete 3GPP band. Herein, a frequency band typically contains multiple subbands associated with carriers carrying information according to different communication technologies and standards such as GSM, UMTS, LTE or the like.
Repeater systems, in general, are constituted to receive a signal, for example, from a base station to process the signal, to amplify the signal and to retransmit it into a coverage area to provide (enhanced) coverage for example in an obstructed area such as within a building or within a movable object such as a train. The repeater system herein receives, in a downlink direction, a signal from a telecommunication network, for example from a base station, processes the signal and retransmits it towards a user equipment (UE), in particular a mobile station. In an uplink direction, in turn, the repeater system receives a signal from a user equipment, processes the signal and transmits it towards a base station.
The repeater system provides coverage in a coverage area via an air interface. Just as well the repeater system may be coupled to one or multiple base stations of one or multiple operators via an air interface, wherein it also is possible that the repeater system is linked to base stations in a wire-bound fashion and is fed by the base stations via suitable communication links such as coaxial cables.
To retransmit RF signals into the coverage area (in the downlink direction towards a user equipment), the RF signals need to be power amplified. A power amplifier herein, generally, shall be used in the linear regime in which the RF power linearly relates to the input power. However, (active and passive) components of radio transmitters are imperfect in that they are not perfectly linear, leading to the generation of unwanted spectral products within and outside of a wanted frequency band. In a downlink direction, a power amplifier may, for example, produce intermodulation products in subbands associated with uplink carriers, leading to unwanted signals in such uplink subbands, which may be problematic because uplink signals received from a user equipment may be weak.
In wireless radio systems requirements exist to define so-called “key performance indicators” (KPI) for wireless transmission. A baseline criterion for the design of a radio amplifier is to meet the so-called “spurious emission limit” substantially for an entire frequency band of use, for example a 3GPP frequency band. This spurious emission limit can generally be achieved by a suitable design of the radio amplifier, for example by applying linearization techniques for linearizing the transfer curve of a power amplifier (such as a digital pre-distortion technique).
A more stringent spurious emission limit is defined as the so-called “co-existence limit”, enabling to operate different 3GPP compliant radio transmitters in the same geographical area (for this it is assumed that a reasonable distance between radio transmitters exists). Yet another limit is defined as the so-called “co-location” limit, which allows to install radio transmitters in the same location or even sharing the same antenna.